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Code	Meaning and what to check
01	Illegal Function — the device doesn't support that function code. Most often you used FC03 on an input register or FC04 on a holding register. Check the register map.
02	Illegal Data Address — the address doesn't exist on this device. Usually an off-by-one between conventional address and offset, or a register that only exists on a different model.
03	Illegal Data Value — the value is out of range for that register, or the quantity requested exceeds what the device allows in one transaction.
04	Slave Device Failure — an unrecoverable error inside the device while processing the request.
05	Acknowledge — request accepted but will take a long time. The master should poll for completion rather than time out.
06	Slave Device Busy — device is processing a long command. Retry later.
08	Memory Parity Error — used with file record functions; indicates a parity error in the device's memory.
0A	Gateway Path Unavailable — a gateway could not allocate a path. Usually means the gateway is misconfigured.
0B	Gateway Target Failed to Respond — the gateway is fine, the downstream device isn't answering. Check the field side, not the gateway.

How to use it

When a request fails, the slave returns the original function code plus 0x80 — so a failed FC03 comes back as 0x83.
The byte after that is the exception code below.
Codes 01, 02 and 03 cover the vast majority of what you'll see in the field.

Go deeper

These tools come out of the book — a field reference for BACnet, Modbus, LonWorks, OPC UA and the diagnostic method that works across all of them.

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Modbus Exception Codes